Adds 'cluster' as a field in the config.

This allows named clusters to be setup via saltcli and the configuration
file. A named cluster means that various configuration that impacts
global state, such as vhost names, globally unique identifiers, etc can
all be established without having to use a user data grain or some other
complex data passing scheme.
Merge pull request #3 from orchestrate-io/add_cluster_opts

Adds 'cluster' as a field in the config.